Biological tissue, such as the human brain, can be stimulated non-invasively when attempting to diagnose a symptom or to achieve a therapeutic effect. Using conventional techniques, it is possible to stimulate biological tissue by virtue of inducing an electric field in the tissue. The technique of magnetic stimulation accomplishes this by means of brief pulses of changing magnetic field. Various apparatuses and methods for providing biological tissue with magnetic stimulation are disclosed, e.g., in publications:                U.S. Pat. No. 4,940,453        U.S. Pat. No. 5,766,124        U.S. Pat. No. 6,132,361 and        U.S. Pat. No. 6,086,525.        
Transcranial magnetic stimulation (TMS) offers a risk- and pain-free method of stimulating the human brain. For instance, stimulation of the motor cortex triggers neuronal signals that travel from the stimulated cortex to pyramidal cell fibers, and via peripheral fibers to the muscles, leading finally to the contraction of the muscles. The contraction of the muscle can be detected and its intensity measured by using an EMG device. The conduction time and the administrated stimulus intensity compared to the EMG response strength provide information that can help diagnosis of neurological diseases and trauma.
In addition to diagnostic uses, TMS has several potential therapeutic applications in diseases or disorders, such as depression, stroke and chronic pain.
In a typical TMS device, a capacitor is first charged to 1-3 kV. The maximum energy of the capacitor is normally between 300 and 500 J. The capacitor is then quickly discharged through an induction coil having small inductance of approximately 5-30 μH. The quick discharge leads into a strong brief current pulse, whose peak current can be 2-10 kA over pulse duration of 200-400 μs. In therapeutic use, magnetic impulses are administered as a sequence of pulses at different pulse repetition rates. Typically the pulse repetition rates between 0.1 and 20 Hz, but rates as high as 100 Hz have already been tested. The number of pulses fired during a treatment session is typically 2 to 5,000.
Coils suitable for above-mentioned magnetic stimulation are described in the following patent publications:                U.S. Pat. No. 6,086,525;        WO 0232504,        GB 2415632,        U.S. Pat. No. 7,367,936,        U.S. Pat. No. 6,663,556,        U.S. Pat. No. 6,179,770,        GB 2261820,        US 2008177128,        EP 1912699, and        U.S. Pat. No. 6,503,187.        
However, conventional TMS coils feature considerable disadvantages. An essential problem is that they heat up very easily due to high current peaks at relatively high repetition rates. The heat problem causes discomfort for the patient and operator of the coil. Existing coils are resistive, typically wound from copper wire and enclosed in plastic casing. The strong current pulse with peak of several kiloamps results in ohmic losses and corresponding Joule heating in the resistive coil. At maximum pulse intensities the resistive losses are typically greater than 10 J/pulse. At a pulse rate of 10 Hz, this adds up to 100 W of heat power originating from the copper wires alone. The heat then transfers to the plastic casings and onto the patient. On the other hand, safe operation (cf. international safety standard IEC60601-1 on medical devices) of transcranial magnetic stimulation limits the maximum surface temperature of the coil to 41° C. or 106° F. To obtain maximal efficacy, the coil is manufactured out of copper wires that are as close to the head as possible, which requires the casing to be thin and the coppers close to the case surface.
The temperature of the coil and its casing depends on the heat capacities and quantities of the materials. Adding material within the coil will reduce the temperature for the same amount of heat energy. The same is achieved by using materials with high heat capacity. Heat can be absorbed into plastic casing material of the coil, or extra epoxy. However, the coil must be light and small also for usability improvement purposes, since the operator may have to handle the coil for rather long periods of time. Accordingly, adding extra weight to the device is not advantageous. Another problem with this solution is that plastic conducts heat slowly and the heat capacity of plastics is rather low compared to more progressive cooling means.
On the whole, several attempts have been made to address the heating issue by cooling the coil by circulating air or fluid substance near the source of the heat. The circulating substance can either stay within the coil or it can be lead out of the structure to exchange heat. For example, publication U.S. Pat. No. 6,179,770 discloses an arrangement of circulating air near the coil for cooling the structure of the device. Another resolution is to circulate liquids as disclosed in publication US 2006004244 and by Nielsen JF (A new high-frequency magnetic stimulator with an oil-cooled coil. J Clin Neurophysiol 1995; 12:460-467). The structure can also be cooled by absorbing heat into static liquid or solid material within the structure.
Since fluid coolants have high heat capacity, they are in principle suitable for cooling purposes. In particular, water has superior volumetric heat capacity, i.e. Joules absorbed per volume. However, the use of liquid coolants makes the device considerably complex and creates a risk of spilling the coolant on the patient in the event of a broken coil casing. Another problem with liquids is that they conduct heat relatively poorly from the coil to its surroundings. Air-cooling is not preferred either, because of increased complexity and also excess noise emanating from the necessary cooling blowers.
Metals would be suitable materials for conducting heat away from the coil because they have high heat conductivity, i.e. they are able transfer heat quickly from one place to another. However, metals have not been used in close proximity of TMS coils, because the changing magnetic field of the coil induces eddy currents in metals that will heat them up. There will also be a force between the primary current in the TMS coil and the eddy current. The force can create a considerable risk and break the construction. The force also causes loud audible noise pulses, which are harmful for occupants of the operating room.
Currently there are no known coil solutions featuring electrically conducting non-ferromagnetic material in the immediate vicinity of the coil coppers. Ferromagnetic materials have, however, been used for the purpose of providing a magnetic core to improve efficiency of the induction coil as is disclosed in publications WO 91/04071 and WO 2007/016279.